Bandpass filters are widely used in communications systems for frequency division multiplexing, to reduce extraneous noise, for impedance matching and the like. At microwave frequencies (roughly 3 to 30 GHz) and at millimeter-wave frequencies (roughly 30 to 300 GHz), electrical signals are often transported by transmission lines in the form of waveguides, which are elongated metal tubes, often having a rectangular or circular cross section. The signals propagate within the tube defined by the conductive walls. Waveguide filters may be implemented with a variety of structures, including conductive diaphragms partially closing off the waveguide with symmetrical or asymmetrical windows, metallic posts and rings. At microwave and millimeter-wave frequencies, these structures may be difficult to fabricate with the accuracy required to achieve the desired frequency response. An article entitled "The Design Of A Bandpass Filter With Inductive Strip-Planar Circuit Mounted In Waveguide" by Konishi, attempts to reduce the fabrication problems with a structure consisting of a metal sheet with appropriate patterns that is inserted into the middle of a waveguide parallel to the E plane. As described therein, the metal sheet includes a plurality of fenestrations or windows which have vertical dimensions equal to the full height of the waveguide. An article entitled "Theory And Design Of Low-Insertion Loss Fin-Line Filters" by Arndt et al., published in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-30, No. 2, February 1982, describes a filter in a waveguide-like structure which includes a dielectric substrate onto which metal strips or posts are bonded which define windows. Methods are given for calculation of the frequency response. When such filters are fabricated, unavoidable tolerances and approximations involved in the calculations result in filters having characteristics which are not at the desired frequency. Furthermore, it may be desirable for test purposes to have the ability to select the bandpass frequency for such purposes as a frequency scanning receiver.